bpo-31231: Fix pythoninfo in Travis config (#3134)

bpo-31231, bpo-30871: Replace "./python -m test.pythoninfo" with
"make pythoninfo", since macOS uses ./python.exe.

Author: vstinner

.travis.yml

@@ -88,7 +88,7 @@ before_script:
         echo "$changes"
         exit 1
       fi
-      ./python -m test.pythoninfo
+      make pythoninfo
 
 script:
   # Using the built Python as patchcheck.py is built around the idea of using